Many condensation polymers of phosphorus containing compounds have important industrial and commercial applications.
Phosphoric acid and phosphoric acid salts are capable of dimerizing, trimerizing and in general polymerizing to form commercially useful compounds. The condensation products contain at least one P--O--P moiety which is formed by the removal of the constituent of water from the phosphoric acid or the phosphoric acid salt monomer.
The process that is used commercially to effect this chemical reaction is known as condensation polymerization since water is generated as the monomer molecules join together. With the exception of enzymatic action on phosphates to form condensed phosphates in living organisms, the reaction is usually driven by the application of heat which causes the water that is formed in the reaction to be vaporized, and allows it to be separated from the reaction zone as steam.
In the condensation of phosphoric acid salts, the heat is usually applied by heated air generated by the combustion of fossil fuels. Hot air can be applied in various ways, for example, in a spray dryer, or in rotating kilns. The heat for the reaction may also be provided by radiant heat from the combustion of fossil fuels. In addition, heat transfer across the walls of metallic equipment from a liquid to the phosphate can be used. To aid in heat transfer, mixing of the phosphates is often necessary, usually by rotating a bed of the phosphate. The polymerization can also be effected by other methods, such as spraying directly into a flame from the combustion of fossil fuels. This partially avoids problems associated with heat transfer, but is difficult to control.
All commercially used methods of condensing phosphoric acid salts suffer from heat transfer restrictions requiring the use of large temperature differences between the heat transfer fluid and the salt being heated. For example, in the preparation of sodium hexametaphosphate, a combination of hot air and radiant heat generated by the combustion of fossil fuels is commonly used to heat a molten pool of phosphate. The heat transfer is typically poor and often as much as 80% of the heat input is wasted.
Condensed phosphoric acid is conventionally prepared either by the combustion of elemental phosphorus or by direct or indirect heating of phosphoric acid with hot gases. Elemental phosphorus is burned by reaction with oxygen followed by dissolution of the phosphorus oxides in more dilute phosphoric acid. In this process, the condensed phosphoric acid is produced without the generation of water from phosphoric acid and this process is not a condensation polymerization process. The production of elemental phosphorus is an expensive process involving the reduction of phosphate rock in a high temperature furnace. Due to inefficiencies in the process, a large amount of energy is wasted in the production of calcium silicate by-products.
The condensation polymerization of phosphoric acid is known to be possible. Using methods involving heat exchange surfaces, the maximum concentration that can be obtained is typically 100-105% expressed as the monomer H.sub.3 PO.sub.4. These methods are limited by poor heat transfer at the heat transfer surface either due to precipitation of highly polymerized phosphoric acid or to the very viscous nature of the product. This reduces the rate of heat transfer to the bulk and results in a high degree of polymerization at the heat transfer surface which forms an insulating layer. For this reason, very large temperature differences between the heating fluid and the bulk of the acid are necessary to drive heat into the bulk of the acid.
Direct heating by submerged combustion devices or other devices using hot combustion gases directly can also supply the heat necessary to cause phosphoric acid to polymerize, but this suffers from limitations. Firstly, heat transfer is poor as the acid becomes more concentrated and as a result, very highly polymerized phosphoric acid results in the region close to the flame. Combustion gases containing impurities may contaminate the acid, and there is significant entrainment of acid droplets which requires expensive and sophisticated scrubbing systems. Also, there is a limitation on the ultimate strength of acid that can be obtained since the combustion fossil fuels leads to the formation of water vapour which suppresses the condensation reaction. Concentrations of approximately 105% are possible if the air used to fire the burner is dried.
A method of heating phosphoric acid by direct electrical resistance in an electrically conductive carbon containing apparatus has been described in U.S. Pat. No. 4,296,082. This process can be used to purify impure phosphoric acid through either the volatilization of impurities, the precipitation of impurities as the acid concentrates, or by their absorption onto the carbon particles resulting from the charring of organic impurities present in the phosphoric acid. The solids can be removed from the acid by filtration.
All of these processes for producing condensation polymers of phosphorus containing compounds suffer from the disadvantage that they are not energy efficient, and are thus very expensive.
Microwave heating differs from conventional forms of heating in that the heating occurs within the volume of the sample to be heated rather than at the surface. There is therefore no heat transfer medium required. The energy is transferred continuously to the entire volume of the material with high power densities. Microwaves themselves do not contain heat. Heat is generated internally by coupling of the internal motions of the atoms and molecules in the material heated to the electromagnetic field of the microwave radiation. Penetration of microwave radiation into the volume of the material to be heated can lead to high internal temperatures and temperature gradients are usually directed outwards from the centre of the material being heated. This assists in internal mass and heat transfer without any other motive force and thus there is no requirement for induced mixing of the material.
However, microwave radiation is not generally used to provide heat for industrial chemical processes as it has a reputation of being prohibitively expensive. Furthermore, it is not possible to predict in advance from a consideration of the chemical and physical properties of a material whether it will absorb sufficient microwave radiation to become hot enough to effect a chemical transformation. For example, using microwave radiation under the same conditions, Fe.sub.3 O.sub.4 can be heated to 510.degree. C. in two minutes, whereas Fe.sub.2 O.sub.3 reaches a temperature of only 88.degree. C. after thirty minutes of heating.
Microwave radiation has been used to heat dilute acids, including sulphuric acid and phosphoric acid, in order to remove free water and obtain a more concentrated acid. For example, in U.S. Pat. No. 4,671,951 to Masse, a process is disclosed for concentrating and purifying waste sulphuric acid using microwave radiation to remove free water. In this process, the microwave radiation is used to heat and remove free water, but not to effect a condensation polymerization reaction. Also, Chang in U.S. Pat. No. 5,451,302 discloses a process for the concentration of phosphoric acid using microwave heating to evaporate water. This process does not show any chemical change. Neither of these patents use microwave radiation for effecting a chemical reaction.